Continuous, carbon-nanotube-reinforced polymer precursors and carbon fibers

ABSTRACT

The present invention relates to a continuous, carbon fiber with nanoscale features comprising carbon and carbon nanotubes, wherein the nanotubes are substantially aligned along a longitudinal axis of the fiber. Also provided is a polyacrylonitrile (PAN) precursor including about 50% to about 99.9% by weight of a melt-spinnable PAN and about 0.01% to about 10% of carbon nanotubes. Other precursor materials such as polyphenylene sulfide, pitch and solution-spinnable PAN are also provided. The precursor can also include a fugitive polymer which is dissociable from the precursor polymer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to continuous carbon nanofiberstructures including carbon nanotubes and polymer (especiallypolyacrylonitrile) precursors including an acrylonitrile-containingpolymer and carbon nanotubes.

2. Description of Related Art

It is well known that, as a general rule, as the diameter of carbonfibers is decreased, strength generally increases. The reasons for thisare usually ascribed to improved molecular orientation (e.g., increasedgraphitic structure) and to a reduction in the number of flaws due tothe improved quality of the cross-sectional filament structure. At theextreme of the continuum lie carbon nanotubes, which ideally are fullygraphitic without flaws in the structure of the walls. However, therealization of the potential of the mechanical benefits of thesematerials is hindered by the requirement of having to transfer loadalong the fiber length between fibers via mechanical entanglementscaused by frictional and van der Waal's interactions between the carbonnanotubes themselves and between adjacent fibers through shear couplingsuch as from a matrix resin.

Currently, continuous carbon fibers with nanoscale features are notavailable except on the research level. Most carbon fibers withnanoscale features are either carbon nanotubes or carbon nanofibers.Carbon nanofibers are generally vapor-grown or electrospun. Vapor-growncarbon fibers typically comprise a range of lengths and are notcontinuous. By contrast, electrospun carbon fibers can be madecontinuously. However, there are many shortcomings to electrospinning.

In electrospinning an electric field is generated between an oppositelycharged polymer fluid and a fiber-collection ground plate. A polymersolution is added to a glass syringe with a capillary tip. As theelectrical potential is increased, the charged polymer solution isattracted to the screen. Once the voltage reaches a critical value, thecharge overcomes the surface tension of the polymer cone formed on thecapillary tip of the syringe and a jet of ultrafine fibers is produced.As the charged fibers are splayed, the solvent quickly evaporates andthe solidified fibers are accumulated randomly on the surface of thecollection screen. This results in a nonwoven mesh of nano to micronscale fibers. Varying charge density, polymer solution concentration andthe duration of electrospinning can control fiber diameter and meshthickness.

The first problem with electrospinning is related to the difficulty incollecting and collimating the fibers in order to handle them as orderedfibers. Currently, it is only possible on length scales of severalinches to one foot. Second, and perhaps more important, electrospinningheads may not be placed in too close of proximity to each other as theelectric fields emanating from each head can interfere with the other.Due to this limitation, in order to produce a large number of fiberends, a commercially impractical area would be required to accommodatethese on a production floor. Customarily, a large number of fiber endsare needed because, in order to approximate the typical 3-24000 filamentends (5-10 microns in diameter) present in commercial carbon tows incross-sectional area, somewhere between 1000 and 10,000 times as manyends would be needed, thereby necessitating several millionelectrospinning heads to achieve this goal. For large-volume productionof continuous fibers, this becomes untenable.

New research suggests that polymer/carbon nanotube composite films andfibers could potentially provide materials having improved tensilestrength. To date, however, only spinning of carbon nanotubes into yarnsand direct electrospinning of nanocarbon fibers have been demonstratedfor continuous nanocarbon fibers. Prior to the present invention, nowork has been demonstrated with carbon fibers with nanoscale features.

As such, there remains a need for continuous carbon fibers withnanoscale features. More specifically, there remains a need forcontinuous carbon-nanotube-reinforced carbon fibers with nanoscalefeatures. Additionally, there remains a need for a method of producing acontinuous carbon-nanotube reinforced carbon fiber with nanoscalefeatures. More specifically there remains a need for a cost-effectivemethod of producing a large-volume of continuous carbon-nanotubereinforced carbon fibers with nanoscale features such that their use incomposites-based, primary load-bearing structures such as for aircraftis practical. Moreover, there remains a need for both melt-spinnable andsolution-spinnable methods for producing continuous,carbon-nanotube-reinforced carbon fibers with nanoscale features.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies at least some of the aforementionedneeds by providing continuous, carbon fibers with nanoscale structuresthat are reinforced with carbon nanotubes. In one embodiment, the carbonfiber comprises carbon and carbon nanotubes, wherein the nanotubes aresubstantially aligned along a longitudinal axis of the fiber. Thecarbon-nanotube-reinforced carbon fibers with nanoscale features cancomprise either solid or hollow fibers. In another embodiment, a carbonfiber can include numerous internal hollow fibers bundled within aresultant filament. Such embodiments can include a honeycomb-like crosssection. As such, these embodiments can comprise an overall resultantfilament with nanoscale wall thicknesses between adjacenthollow-cylinder-like portions of the honeycomb cross section.

In another aspect, the invention provides a polyacrylonitrile (PAN)precursor. According to embodiments of the present invention, the PANprecursor can comprise about 50% to about 99.9% by weight of amelt-spinnable PAN and about 0.01% to about 10% of carbon nanotubes. Incertain embodiments, the PAN precursor includes a fugitive polymer whichis dissociable from the melt-spinnable PAN.

In yet another aspect, the invention provides a method of forming acontinuous carbon-nanotube-reinforced carbon fiber structure withnanoscale features. One method according to an embodiment of the presentinvention includes providing a resin mixture comprising from about 50 toabout 99.9% of a melt-spinnable-polyacrylonitrile (PAN) and from about0.05% to about 10% of carbon nanotubes. The resin mixture is extrudedand fed into a spin-pack assembly capable of producing fibers comprisingnanoscale dimensions and substantially aligning the carbon nanotubeswith the PAN. Next, the resulting PAN/carbon nanotube fibers areoxidized by sufficiently heating the fibers. The oxidized fibers can besubjected to a carbonization process comprising the heating of theoxidized fibers to a temperature ranging from about 600 to about 3000°C. In certain embodiments, the method also includes providing a fugitivepolymer which is dissociable from the melt-spinnable PAN. Preferably,the two components (i.e., the resin mixture and the fugitive polymer)are separately extruded and fed into a spin-pack assembly capable ofproducing multi-component fibers comprising nanoscale dimensions in anislands-in-a-sea configuration and substantially aligning the carbonnanotubes with the PAN. Preferably, the fugitive polymer is removed byextraction or the like.

In another embodiment of the invention, solution-spinnable orgel-spinnable PAN can be used in a similar manner to that describedabove.

Likewise, in certain embodiments other polymers, such as pitch(especially mesophase pitch) or polyphenylene sulfide may be substitutedfor the melt-spinnable PAN in the melt-spinning described herein.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE FIGURE(S)

Having thus described the invention in general terms, reference will nowbe made to the accompanying figures, wherein:

FIG. 1 illustrates a cross sectional view of an “islands-in-a-sea” PANprecursor having multiple islands comprising a PAN and a sea comprisinga fugitive polymer;

FIG. 2 illustrates a cross-sectional view of a carbon nanofiberaccording to one embodiment of the present invention, wherein the fiberhas a honeycomb-like cross section where the islands have been removedfrom an islands-in-a-sea filament to leave a continuous honeycomb-likecross section;

FIG. 3 illustrates a cross-sectional view of a carbon nanofiberaccording to another embodiment of the present invention, wherein thefiber has a honeycomb-like cross section where the islands have beenremoved from an islands-in-a-sea filament to leave a continuoushoneycomb-like cross section.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, the present invention comprises a continuouscarbon-nanofiber structure comprising carbon and carbon nanotubes,wherein the nanotubes are substantially aligned along a longitudinalaxis of the fiber. In certain embodiments, the continuous carbonnanofiber can be made directly from a melt-spinnable polyacrylonitrile(PAN) precursor. Other embodiments can be made by utilizing a spin packcapable of producing multi-component fibers. In preferred embodiments,the resultant multi-component fibers comprise an islands-in-the-seaconfiguration, wherein the multi-component fibers comprise a PAN and afugitive polymer which can be dissociable from the melt spinnable PAN.

As used herein, the term “fugitive polymer” refers to compounds that canbe extracted out of a multi-component fiber after melt spinning, but atany one of several points of the fiber-making process. In general,multi-component fibers are formed of two or more polymeric materialswhich have been extruded together to provide continuous contiguouspolymer segments which extend down the length of the fiber.

The fugitive polymer, for example, can include water-soluble polymers,such as but not limited to, polyvinyl alcohol, polyethylene oxide,polyacrylamide, polylactic acid, water-soluble copolyester resins, andcopolymers, terpolymers, and mixtures thereof. Other additives, such asbasic or ionic compounds, may be added to an aqueous bath to aid indissolution, as is known in the art. For example, polylactic acid can besoluble in caustic aqueous solutions. Alternatively the fugitive polymercan include a solvent-extractable polymer, such as polystyrene. Incertain embodiments of the invention, the fugitive polymer is typicallyextracted from the multi-component fiber using a suitable solvent aftermelt spinning but before heat treatment to carbonize the PAN component.

Alternatively, the fugitive component can include a polymer which formsa char upon heat treatment such as that generally associated with thecarbonization of PAN polymers. The charred residual component can thenbe extracted or removed from the multi-component fibers generally usingmechanical means, such as impingement by high-pressure air or waterjets.

The term “fiber” as used herein means both fibers of finite length, suchas conventional staple fiber, as well as substantially continuousstructures, such as filaments, unless otherwise indicated.

In general, the melt-spinnable PAN and fugitive-polymer components arechosen so as to be mutually incompatible. In particular, the polymercomponents do not substantially mix together or enter into chemicalreactions with each other. Specifically, when spun together to form acomposite fiber, the polymer components exhibit a distinct phaseboundary between them so that substantially no blend polymers areformed, preventing dissociation. The various components should have theappropriate solubility characteristics, such that the fugitive polymeris soluble in solvent (if removed using a solvent-extraction process),while the insoluble polymer must be capable of withstanding theextraction of the fugitive polymer without detriment. In addition, abalance of adhesion/incompatibility between the components of thecomposite fiber is considered highly beneficial. The componentsadvantageously adhere sufficiently to each other to allow thepre-extracted multi-component fiber to be subjected to conventionaltextile processing such as winding, twisting, weaving, knitting orcarding without any appreciable separation of the components, if sodesired. Conversely, the polymers should be sufficiently incompatible sothat adhesion between the components is sufficiently weak, so as toprovide ready dissolution during the extraction process.

According to preferred embodiments of the present invention, amelt-spinnable PAN is subjected to melt spinning. As used herein, theterm “polyacrylonitrile (PAN)” polymer includes polymers comprising atleast about 85% by weight acrylonitrile units (generally known in theart as acrylic or polyacrylonitrile polymers). This term as used hereinalso includes polymers which have less that 85% by weight acrylonitrileunits. Such polymers include modacrylic polymers, generally defined aspolymers comprising 35-85% by weight acrylonitrile units and typicallycopolymerized with vinyl chloride or vinylidene chloride. Preferably,the polyacrylonitrile polymer has at least 85% by weightpolyacrylonitrile units. Other polymers known in the art to be suitableprecursors for carbon and graphite fibers, such as polyvinyl alcohol,aromatic polyamides, or poly(acetylenes), may be suitable for use in thepresent invention, if capable of extrusion by melt spinning.

Exemplary melt-processable polyacrylonitriles are described in U.S. Pat.Nos. 5,602,222, 5,618,901 and 5,902,530 the entire disclosure of each ofwhich is hereby incorporated by reference. Such polymers arecommercially available, for example, from BP Chemicals as the “Amlon™”acrylic polymers, “Barex®” acrylic polymers, and the like. SeeInternational Fiber Journal, p. 42, April 1998, hereby incorporated byreference in its entirety. In one preferred embodiment, the PAN polymercomprises Amlon™

Melt-processable/spinnable PANs are particularly attractive for use inthe present invention because they are excellent precursors for theformation of carbon fibers. In addition, melt-processable PANs exhibitadequate heat resistance, with a melting point of approximately 185° C.Polyacrylonitrile fibers also exhibit good tensile strength andresilience. In addition, polyacrylonitrile fibers are especiallyadvantageous in those embodiments employing dissolution as a means ofextraction, because polyacrylonitrile possesses superior water andchemical resistance, thus allowing a wide range of solvents to beemployed in the dissolution of the fugitive component.

In addition to containing acrylonitrile monomer, melt-processablepolyacrylonitrile polymers can also include olefinically unsaturatedmonomer. The acrylonitrile olefinically unsaturated polymer ispreferably made up of about 50 weight % to about 95 weight %, preferablyabout 75 weight % to about 93 weight %, and most preferably about 85weight % to about 92 weight %, of polymerized acrylonitrile monomer, andat least one of about 5 weight % to about 50 weight %, preferably about7 weight % to about 25 weight %, and most preferably about 8 weight % toabout 15 weight %, of polymerized olefinically unsaturated monomer.

In certain embodiments, the olefinically unsaturated monomer can includeone or more of an olefinically unsaturated monomer with a C═C doublebond polymerizable with an acrylonitrile monomer. The olefinicallyunsaturated monomer can be a single polymerizable monomer resulting in aco-polymer, or a combination of polymerizable monomers resulting in amulti-polymer. The choice of olefinically unsaturated monomer or acombination of monomers can depend upon the properties desired to impartto the resulting fiber and its end use. The olefinically unsaturatedmonomer generally includes, but is not limited to, acrylates such asmethyl acrylates and ethyl acrylates; methacrylates, such as methylmethacrylate; acrylamides and methacrylamides and each of theirN-substituted alkyl and aryl derivatives, such as acrylamide,methacrylamide, N-methylacrylamide, N,N-dimethyl acrylamide; maleic acidand its derivatives, such as N-phenylmaleimide; vinylesters, such asvinyl acetate; vinylethers, such as ethyl vinyl ether and butyl vinylether; vinylamides, such as vinyl pyrrolidone; vinylketones, such asethyl vinyl ketone and butyl vinyl ketone; styrenes, such asmethylstyrene, styrene and indene; halogen-containing monomers, such asvinyl chloride, vinyl bromide, and vinylidene chloride; ionic monomers,such as sodium vinylsulfonate, sodium styrenesulfonate, and sodiummethyl sulfonate; acid containing monomers such as itaconic acid,styrene sulfonic acid and vinyl sulfonic acid; base-containing monomers,such as vinyl pyridine, 2-aminoethyl-N-acrylamide,3-aminopropyl-N-acrylamide, 2-aminoethylacrylate,2-aminoethylmethacrylate; and olefins, such as propylene, ethylene,isobutylene. Other monomers, such as vinyl acetate, acrylic esters, andvinyl pyrrolidone, may also be included in conventionalpolyacrylonitrile in small amounts, to allow the resultingpolyacrylonitrile fiber to be dyed with conventional textile dyes.Additional properties may also be imparted to melt-processable polymerscontaining significant amounts of acrylonitrile by choosing appropriateco-monomers or blends thereof. For example, the inclusion of styrene inthe polymer results in improved heat distortion; isobutylene improvesthe flexibility; halogen-containing monomers increase the flameresistance of the polymer. Still further, the acrylonitrile polymer caninclude methacrylonitrile monomer. The use of such co-monomers isdiscussed in more detail in U.S. Pat. Nos. 5,602,222 and 5,618,901.

Embodiments of the present invention comprise carbon fibers containingnanoscale structures within their cross section, preferably beingcontinuous, having carbon nanotubes which are substantially alignedalong a longitudinal axis of the fiber due to the geometric constraintsimposed by the spin pack, which is discussed in further detail below.More specifically, the geometric constraints imposed by the choice ofspin pack, according to various embodiments, helps cause the polymermolecules and the carbon nanotubes to become substantially aligned. Invarious embodiments, single-wall or multi-wall carbon nanotubes can beutilized.

Single-wall carbon nanotubes can be made from any known means, such asby gas-phase synthesis from high-temperature, high-pressure carbonmonoxide, catalytic vapor deposition using carbon-containing feedstocksand metal catalyst particles, laser ablation, arc method, or any othermethod for synthesizing single-wall carbon nanotubes. The single-wallcarbon nanotubes obtained from synthesis are generally in the form ofsingle-wall-carbon-nanotube powder.

In one embodiment, single-wall-carbon-nanotube powder is purified toremove non-nanotube carbon, such as amorphous carbon and metalliccatalyst residues. Metals, such as Group VIB and/or VIIIB, are possiblecatalysts for the synthesis of single-wall carbon nanotubes. Aftercatalysis, the metallic residues may be encapsulated in non-nanotubecarbon, such as graphitic shells of carbon. The metallic impurities mayalso be oxidized through contact with air or by oxidation of thenon-nanotube carbon during purification.

In one aspect, the invention comprises a polyacrylonitrile (PAN)precursor including from about 50% to about 99.9% by weight of a meltspinnable PAN and about 0.01% to about 10% of carbon nanotubes. In oneembodiment the PAN precursor can comprise solid fiber having an outsidediameter ranging from 20 to 1000 nanometers, or 50 to 950, or 100 to900, or 250 to 600 nanometers. In one alternative embodiment, the PANprecursor can comprise hollow fibers having the an outside diameterranging from 20 to 1000 nanometers, or 50 to 950, or 100 to 800, or 250to 600 nanometers. In certain embodiments, the wall thickness of thehollow precursor can range from 10 to 500 nanometers, or from 100 to 400nanometers or from 200 to 300 nanometers. According to anotherembodiment, the wall thickness of the hollow precursor can range from 10to 150 nanometers, or from 20 to 100 nanometers, or 35 to 70 nanometers.In alternative embodiments, the PAN precursors can comprise an outsidediameter from about 1 micron to about 50 microns, or from about 1 micronto about 10 microns, with a wall diameter ranging from 10 to 500nanometers, or from 100 to 400 nanometers, or from 200 to 300nanometers. According to another embodiment, the wall thickness of thehollow precursor can range from 10 to 150 nanometers, or from 20 to 100nanometers, or 35 to 70 nanometers.

In another embodiment, the PAN precursor also includes a fugitivepolymer which is dissociable from the melt-spinnable PAN as discussedabove. In various embodiments, the fugitive polymer can comprise awater-soluble polymer or an organic-solvent-extractable polymer asprovided above. In preferred embodiments, the fugitive polymer cancomprise polylactic acid or a polyester.

In certain embodiments, the PAN precursor comprises anislands-in-the-sea configuration. In various embodiments, the PANpolymer comprises a plurality of solid islands and the fugitive polymercomprises the sea. For example, FIG. 1 illustrates a cross sectionalview of an “islands-in-a-sea” PAN precursor 1 having multiple islands 20comprising PAN, which are surrounded by a sea 30 comprising a fugitivepolymer. The number of PAN polymer islands can be varied as can theirdiameters. For example, the number of islands can range from 50 to 2000,or 300 to 1500, or 500 to 1000 islands. The outside diameter of the PANislands can also be controlled by choice of spin pack. For example, thediameter of the PAN islands can range from 20 to 1000 nanometers, or 50to 950, or 100 to 900, or 250 to 600 nanometers.

In an alternative embodiment, the PAN precursor comprises anislands-in-the-sea configuration comprising a plurality of hollow PANislands with a fugitive polymer comprising the sea. The number of hollowPAN polymer islands can be varied as can their diameters. For example,the number of islands can range from 50 to 2000, or 300 to 1500, or 500to 1000 islands. The outside diameter of the hollow PAN islands can alsobe controlled by choice of spin pack. For example, the diameter of thehollow PAN islands can range from 20 to 1000 nanometers, or 50 to 950,or 100 to 900, or 250 to 600 nanometers. Furthermore the wall thicknessof the hollow PAN islands can be varied to include the dimensionsdescribed above.

In various embodiments, the weight ratio of the PAN to the fugitivepolymer ranges from about 20/80 to about 80/20. Alternatively, theislands-in-the-sea embodiments can also be characterized by theirisland/sea ratio. The island/sea ratio can also range from 20/80 to80/20. In one preferred embodiment, the island/sea ratio ranges from40/60 to 50/50.

In yet another embodiment, the PAN precursor comprises anislands-in-the-sea configuration such that the fugitive polymercomprises a plurality of islands and the PAN (i.e. PAN/carbon nanotubes)comprises the sea. As discussed above, the number of islands and theirdimensions can be controlled or tailored to meet a specific need. Assuch, the diameter of the fugitive polymer islands can range from 20 to1000 nanometers, or 50 to 950, or 100 to 900, or 250 to 600 nanometers.In various embodiments, the weight ratio of the PAN to the fugitivepolymer ranges from about 20/80 to about 80/20. Alternatively, theislands-in-the-sea embodiments can also be characterized by theirIsland/Sea ratio. The island/sea ratio can also range from 20/80 to80/20. In one preferred embodiment, the island/sea ratio ranges from40/60 to 50/50.

Beneficially, PAN-precursor embodiments comprising theislands-in-the-sea configuration can provide environmental benefitssince a large number of fiber ends can be produced from a single PANprecursor. In embodiments comprising a PAN polymer as the sea, thefugitive polymer can be washed away (e.g., extracted from themulti-component fiber) to leave behind a PAN precursor having ahoneycomb-like cross section. Such embodiments include an outer wall anda plurality on integral internal walls. As such, a substantially hollow(e.g., hollow but for the internal PAN walls) PAN fiber can be obtainedwith the benefit of having numerous internal hollow fibers bundledwithin. In various embodiments, the outer wall can comprise numerousshapes. For example, the outer wall can comprise a circle, square, ormulti-nodal outer configuration.

As such, the resulting carbon-nanotube-reinforced PAN fibers of thepresent invention can beneficially be graphitized into structural carbonfibers. Such carbon-nanotube-reinforced PAN fibers can include nanotubedimensions to provide improved properties of over conventional PAN-basedcarbon fibers as well as carbon-nanotube-reinforced, micron-scaledPAN-based carbon fibers previously developed. The PAN precursors canhave the fugitive polymer, if present, washed away, oxidized, andsubjected to a carbonization treatment to produce a carbon fiber.

In one embodiment according to the present invention, a continuouscarbon-fiber structure with nanoscale features is provided. The fibersinclude carbon and carbon nanotubes and have an outside diameter.Notably, the nanotubes are substantially aligned along a longitudinalaxis of the fiber. In one embodiment, the outside diameter ranges fromabout 20 to about 1000 nanometers, or from about 20 to about 750nanometers, or from about 50 to 500 nanometers, or from about 50 to 250nanometers.

According to various embodiments, the carbon-fiber structure cancomprise either a solid or hollow carbon fiber comprising a wall formedof carbon and carbon nanotubes. In these embodiments, the nanotubes arepreferably substantially aligned along a longitudinal axis of the fiber.In preferred embodiments, the fiber structure comprises a percentage ofcarbon nanotubes ranging from about 0.01% to about 10% by weight, orfrom about 0.1% to about 5% by weight, or more preferably from about0.1% to about 1% by weight. The outside diameter of the carbon fiberscan be varied to fit a desired need or to provide desired properties.For instance the outside diameter of the carbon fiber can range from 20to about 1000 nanometers, or from about 50 to about 950, or 100 to about900, or 250 to about 700 nanometers.

In certain embodiments, the carbon-fiber structure comprises ahollow-fiber structure comprising a wall formed of carbon and carbonnanotubes. The thickness of the wall can range from range from about 10to about 500 nanometers. In alternative embodiments, the carbon fibercan comprise an outside diameter of the fiber structure comprises fromabout 1 micron to about 50 microns, or from about 1 micron to about 10microns, or from about 100 to about 900 nanometers. As such, embodimentsof the present invention can include solid or hollow continuous carbonfibers with diameters, wall thicknesses, or both in the nanoscale. Suchcarbon fibers may have excellent mechanical properties suitable formanufacture of composite materials using traditional manufacturingprocesses such as laminating, weaving, etc.

As shown in FIGS. 2 and 3, certain preferred embodiments comprise acarbon-fiber structure 50 comprises a substantially hollow fibercomprising an outer wall 60 having an outside diameter and multipleinternal walls 70 defining multiple integral internal hollow fibers 80such that the fiber structure comprises a honeycomb-like cross section.Such embodiments include an outer wall 60 and a plurality on integralinternal walls 70. As such, a substantially hollow (e.g., hollow but forthe internal walls defining multiple internal hollow carbon fiberstherein) carbon fiber can be obtained with the benefit of havingnumerous internal hollow fibers bundled within. For instance, thehoneycomb-like morphology can comprise an overall resultant filamentwith nanoscale wall thicknesses between adjacent hollow cylinder-likeportions of the honeycomb cross section. In various embodiments, theouter wall can comprise numerous shapes. For example, the outer wall cancomprise a circle, square, or multi-nodal outer configuration. Invarious embodiments, the outer wall comprises a thickness ranging fromabout 10 to about 500 nanometers. In certain embodiments, the internalwalls comprises a thickness ranging from about 10 to about 500nanometers, or about 10 to 250 nanometers, or about 50 to about 100nanometers. In such embodiments, each integral internal hollow fiber cancomprise an internal diameter ranging from about 10 to about 100, 200,300, 400 or 500 nanometers. Further, the outside diameter of theresulting fiber can range from about 100 to about 1000, 2000, 3000, 4000or 5000 nanometers. In certain embodiments, the outside diameter canrange from about 1 micron to about 50 microns, or from about 1 micron toabout 10 microns.

According to embodiments of the present invention, a carbon fiber havinga honeycomb-like cross section, wherein a plurality of integral internalhollow fibers are bundled within an outer wall, can comprise an outsidediameter of the outer wall ranging from about 1 micron to about 50microns. Alternatively, the outside diameter can range from about 1micron to about 10 microns or preferably from about 3 to about 10microns. These tows can be graphitized in the conventional manner andsupplied for use in forming composite materials or fabrics for compositematerials. For example, a composite can be formed comprising layers ofsheets comprising continuous carbon fibers with nanoscale featuresaccording to the present invention. Likewise, a prepreg comprising acloth comprising carbon fibers with nanoscale features according thepresent invention can be produced.

In alternative embodiments, other polymers, such as pitch orpolyphenylene sulfide may be substituted for the melt-spinnable PANdescribed herein. Pitch is the name for any of a number of highlyviscous liquids which appear solid at room temperature and include amixture of predominantly aromatic and alkyl-substituted aromatichydrocarbons. Pitch can be made from petroleum products or plants.Petroleum-derived pitch is also called bitumen while pitch produced fromplants is also known as resin. Preferably, the pitch polymer comprises amesophase pitch. When heated, pitch materials form an isotropic mass. Asheating continues, spherical bodies begin to form. The spherical bodiesare of an anisotropic liquid-crystalline nature. These spheres continueto grow and coalesce until a dense continuous anisotropic phase forms,which phase has been termed the “mesophase.” Thus, the mesophase is theintermediate phase or liquid crystalline region between the isotropicpitch and the semi-coke obtainable at higher temperatures. In oneparticular embodiment, the mesophase pitch comprises Mitsubishi ARA24naphthalene-based synthetic pitch or the like. Mesophase pitch suitablefor certain embodiments of the present invention can be extracted fromnatural pitch. For example, mesophase pitch can be solvent extractedfrom isotropic pitch containing mesogens as described in U.S. Pat. No.5,032,250, the contents of which are hereby incorporated by reference.U.S. Pat. Nos. 4,277,324 and 4,208,267 also describe processes forobtaining mesophase pitch by treating isotropic pitch; the contents ofeach are hereby incorporated by reference. An isotropic pitch comprisesmolecules which are not aligned in optically ordered crystals andmesogens are mesophase-forming materials or mesophase precursors. Inother alternative embodiments, polyphenylene sulfide is substituted forthe melt-spinnable PAN. Polyphenylene sulfide is considered as animportant high-temperature polymer because it exhibits a number ofdesirable properties. For instance, polyphenylene sulfides desirablyexhibit resistance to heat, acids and alkalis, to mildew, to bleaches,aging, sunlight, and abrasion. In one alternative embodiment, thecontinuous carbon nanofiber comprises a long chain synthetic polysulfidein which at least 85% to about 99% of the sulfide linkages are attacheddirectly to two aromatic rings. In particular embodiments, a polyarylenesulfide resin composition is substituted for the PAN. For instance, theresin composition can include at least 70 mole % of p-phenylene sulfideunits (e.g., 70 mole % to 100 mole % or 80 mole % to 90 mole %)represented by the following structure:

In such compositions, the balance or remaining 30 mole % can include anycombination of the following bonds:

wherein R is selected from the group consisting of an alkyl or an alkoxygroup having from 1 to 12 carbon atoms, a phenyl group and a nitrogroup. In various embodiments, the resin compositions can also includemetal hydroxides and/or iron oxides. Suitable resin compositions areprovided in U.S. Pat. No. 5,021,497, the contents of which are herebyincorporated by reference.

In another aspect, the invention comprises a method of forming acontinuous carbon nanofiber structure by melt spinning a resin mixturecomprising a melt-spinnable-polyacrylonitrile (PAN) polymer and fromabout 0.01% to about 10% of carbon nanotubes. Methods according toembodiments of the present invention include steps of providing a resinmixture comprising from about 50 to about 99.9% of amelt-spinnable-polyacrylonitrile (PAN) polymer and from about 0.01% toabout 10% of carbon nanotubes. The carbon nanotubes can be dispersedwithin the mixture by mechanical and/or chemical means (e.g.,dispersants or surfactants). The resin mixture can be extruded and fedinto a spin-pack assembly capable of producing fibers comprisingnanoscale dimensions and substantially aligning the carbon nanotubeswith the PAN. In various embodiments, a fugitive polymer which isdissociable from the melt-spinnable PAN is provided to a separateextruder. The resin mixture and the fugitive polymer can be separatelyextruded and fed into a spin-pack assembly capable of producingmulti-component fibers comprising nanoscale dimensions in anislands-in-the-sea configuration and substantially aligning the carbonnanotubes with the PAN polymer. If an islands-in-the-sea configurationis being produced, various methods include extracting the fugitivepolymer from the multi-component fibers to form PAN fibers. These fiberscan be oxidized by sufficiently heating the PAN fibers. Oxidation caninvolve heating the PAN fibers to around 300° C. The PAN polymer changesfrom a ladder structure to a stable ring structure as understood bythose skilled in the art. To form continuous carbon fibers, which arereinforced by carbon nanotubes being substantially aligned therewith,the oxidized PAN fibers are subjected to a carbonization. Carbonizationcan comprise heating of the oxidized fibers to a temperature rangingfrom about 600 to about 3000° C.

In one preferred embodiment, the method comprises feeding an extrudedresin mixture comprising a PAN polymer and carbon nanotubes and afugitive polymer to an appropriately designed spin pack (e.g.,spinneret). In an alternative embodiment, only a resin mixture includinga PAN polymer and carbon nanotubes is fed to the spinneret. Theseembodiments do not produce an islands-in-the-sea configuration, whileembodiments feeding both a mixture including a PAN polymer and carbonnanotubes and a fugitive polymer are suitable for providing anislands-in-the sea-configuration. In one such embodiment, the resinmixture is fed to the spinneret to provide a plurality of PAN/carbonnanotube islands and the fugitive polymer comprises the sea. In oneembodiment, the islands comprise hollow PAN/carbon nanotube fibershaving a wall thickness ranging from about 10 to about 500 nanometers.However, other embodiments can comprise wall thicknesses and othergeometrical dimensions discussed previously. In another embodiment, theresin mixture and the fugitive polymer are fed to the spinneret toprovide a plurality of fugitive-polymer islands and a sea comprising theresin mixture.

Extrusion parameters for making multi-component continuous-filamentfibers comprising a PAN polymer/carbon nanotube mixture and the fugitivepolymer to provide a fiber having nanoscale dimensions according toembodiments described herein can vary depending on the propertiesdesired. In general, however, to form a multi-component fiber, at leasttwo polymers (e.g., PAN/carbon nanotube mixture and the fugitivepolymer) are extruded separately and fed into a polymer-distributionsystem wherein the polymers are introduced into a spinneret plate. Thepolymers follow separate paths to the fiber spinneret and are combinedin a spinneret hole. The spinneret is configured so that the extrudanthas the desired overall fiber cross section (e.g., round, trilobal,etc.). Such a process is described, for example, in Hills U.S. Pat. No.5,162,074, the contents of which are incorporated herein by reference intheir entirety.

In various embodiments of the present invention, a melt-processablepolyacrylonitrile polymer stream and a fugitive polymer stream are fedinto the polymer-distribution system. The polymers typically areselected to have melting temperatures such that the polymers can be spunat a polymer throughput that enables the spinning of the componentsthrough a common capillary at substantially the same temperature withoutdegrading one of the components.

Following extrusion through the die, the resulting thin fluid strands,or filaments, remain in the molten state for some distance before theyare solidified by cooling in a surrounding fluid medium, which may bechilled air blown through the strands. Once solidified, the filamentsare taken up on a godet or other take-up surface. For continuousfilaments, the strands are taken up on a godet that draws down the thinfluid streams in proportion to the speed of the take-up godet.

Continuous-filament fiber may further be processed into staple fiber. Inprocessing staple fibers, large numbers, e.g., 10,000 to 1,000,000strands, of continuous filament are gathered together followingextrusion to form a tow for use in further processing, as is known inthat art. The use of such tows is likewise known in continuous-filamentapplications, as well. A finish solution may optionally be applied, toaid in fiber processing, as is known in the art. Such finish should bechosen so as not to interfere with downstream processes such asextraction and various heat treatments.

According to certain embodiments, a heightened molecular alignment canbe achieved while producing the carbon-nanotube-reinforced fibers due tothe geometric constraints imposed during spinning. These constraints arepreferably greater than those realized when producing larger-diameterfibers. Additionally, the spinneret can be designed to allow for thetailoring of filament diameter and/or wall thickness. As such, a wholerange of properties may be achieved.

While multi-component fibers are not new per se, polymer-distributiontechnology allowing the economical production of micro- and nano-sizedfibers is relatively new. Spin-pack hardware components havehistorically been manufactured by conventional methods such as millingand drilling. Alternatively, a modern system can use techniques similarto printed-circuit-board technology to manufacture the spin-packcomponents. These precise components are then used to accuratelydistribute polymers in the extremely small area available in the spinpack. Such spin packs allow for the economical and practical productionof micro- and nano-sized fibers. Such spin-packs can be provided byHills, Inc. (W. Melbourne, Fla.).

Preferably, continuous carbon fibers with nanoscale features structuresincluding carbon nanotubes substantially aligned therein according toembodiments of the present invention are produced by utilizing a spinpack having a distribution system that provides a level of precision toenable the production of nanoscale features within fiber/filament crosssections, especially nanoscale islands-in-a-sea type fibers. Preferably,the geometrical constraints imposed by the precise distribution systemof such spin packs substantially align the carbon nanotubes along alongitudinal axis of the fiber. More specifically, the geometricconstraints imposed by the choice of spin pack helps cause use thepolymer molecules and the carbon nanotubes to become substantiallyaligned within the PAN. For instance, carbon nanotubes can besubstantially aligned within the PAN throughout substantially the entirecross section of a PAN precursor having a honeycomb-like cross section.Likewise, carbon nanotubes can be substantially aligned withsubstantially the entire cross section of a continuous carbon fiber withnanoscale features having a honeycomb-like cross section

According to various alternative embodiments, the melt-spinnable PAN canbe replaced with other polymers such as pitch (preferably mesophasepitch) or polyphenylene sulfide. In one such embodiment, carbonnanotubes are blended in to molten pitch at or slightly above itssoftening temperature. The blend is then heated to an extrusiontemperature which can be about 20° C. to about 30° C. above thesoftening temperature and a pitch fiber is extruded by melt spinning asdiscussed herein. The pitch-based fiber, having carbon nanotubessubstantially aligned along a longitudinal axis of the fiber, is nextoxidized and then carbonized.

In additional alternative embodiments, a solution of 10 to 50% by weightof a PAN in a solvent, preferably a highly polar solvent such as sodiumthiocyanate or dimethylacetamide, is utilized. To this solution, carbonnanotubes are added so that the resulting PAN precursor includes anamount of carbon nanotubes ranging from about 0.01% to about 10% byweight of the PAN precursor. The PAN-based solution, having carbonnanotubes added thereto, is extruded through a multi-hole spinneret intoa coagulation bath. As the solution exits the multi-hole spinneret andenters the bath it precipitates forming a multifilament bundle. Due tothe geometric constraints of the spinneret, the carbon nanotubes aresubstantially aligned along a longitudinal axis of the PAN precursor.The bundle is washed to remove excess solvent, oxidized, and thencarbonized to provide a continuous carbon nanofiber structure of carbonand carbon nanotubes. Beneficially, the nanotubes are substantiallyaligned along a longitudinal axis of the fiber. In various embodiments,the solution of PAN can comprise from 10% to 99.9% of a PAN, or 30% to90%, or 40% to 60% by weight of a PAN in a solvent.

According to such solution-spinnable embodiments, solution-spinnablePANs can include any commercially available PAN suitable for solutionspinning, such as a Mitsubishi® PAN (95% PAN, 5% methyl acrylate) orAmlon® (80% PAN, 20% methylacrylate) precursor materials. In someembodiments, the solution-spinnable PAN can be replaced with otherpolymers such as isotropic pitches. Solution-spinnable PANs arecommercially available from Japan Exlan Co., Ltd. (Osaka, Japan).

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. A continuous carbon fiber with nanoscale features comprising carbonand carbon nanotubes, the nanotubes being substantially aligned along alongitudinal axis of the fiber, wherein said fiber includes an outsidediameter.
 2. The continuous carbon fiber with nanoscale featuresaccording to claim 1; wherein the outside diameter ranges from about 20to about 1000 nanometers.
 3. The continuous carbon fiber with nanoscalefeatures according to claim 1; wherein the fiber structure comprises ahollow carbon fiber comprising a wall formed of carbon and carbonnanotubes, the nanotubes being substantially aligned along alongitudinal axis of the fiber.
 4. The continuous carbon fiber withnanoscale features according to claim 3, wherein the fiber comprises apercentage of carbon nanotubes ranging from about 0.01% to about 10% byweight.
 5. The continuous carbon fiber with nanoscale features accordingto claim 3, wherein the fiber comprises a percentage of carbon nanotubesranging from about 0.1% to about 5% by weight.
 6. The continuous carbonfiber with nanoscale features according to claim 3, wherein the fibercomprises a percentage of carbon nanotubes ranging from about 0.1% toabout 1% by weight.
 7. The continuous carbon fiber with nanoscalefeatures according to claim 3, wherein the wall comprises a thicknessranging from about 10 to about 500 nanometers.
 8. The continuous carbonfiber with nanoscale features according to claims 7, wherein an outsidediameter of the fiber structure comprises from about 1 micron to about50 microns.
 9. The continuous carbon fiber with nanoscale featuresaccording to claim 6, wherein an outside diameter of the fiber structurecomprises from about 1 micron to about 10 microns.
 10. The continuouscarbon fiber with nanoscale features according to claim 6, wherein theoutside diameter of the fiber structure comprises from about 100 toabout 900 nanometers.
 11. The continuous carbon fiber with nanoscalefeatures according to claim 1; wherein the fiber structure comprises asubstantially hollow fiber comprising an outer wall having an outsidediameter and multiple internal walls defining multiple integral internalhollow fibers such that the fiber structure comprises a honeycomb-likecross section.
 12. The continuous carbon fiber with nanoscale featuresaccording to claim 11, wherein the outer wall comprises a thicknessranging from about 10 to about 500 nanometers.
 13. The continuous carbonfiber with nanoscale features according to claim 11, wherein theinternal walls comprises a thickness ranging from about 10 to about 500nanometers.
 14. The continuous carbon fiber with nanoscale featuresaccording to claim 11, wherein an outside diameter of the fiberstructure comprises from about 1 micron to about 50 microns.
 15. Thecontinuous carbon fiber with nanoscale features according to claim 11,wherein an outside diameter of the fiber structure comprises from about1 micron to about 10 microns
 16. The continuous carbon fiber withnanoscale features according to claim 11, wherein each integral internalhollow fiber comprises an internal diameter ranging from about 10 toabout 500 nanometers.
 17. A polyacrylonitrile (PAN) precursor,comprising: (a) about 50% to about 99.9% by weight of a melt-spinnablePAN; and (b) about 0.01% to about 10% of carbon nanotubes.
 18. Apolyacrylonitrile (PAN) precursor according to claim 17, furthercomprising a fugitive polymer which is dissociable from themelt-spinnable PAN.
 19. A polyacrylonitrile (PAN) precursor according toclaim 18, wherein the fugitive polymer comprises a water-solublepolymer.
 20. A polyacrylonitrile (PAN) precursor according to claim 18,wherein the fugitive polymer comprises an organic-solvent-extractablepolymer.
 21. A polyacrylonitrile (PAN) precursor according to claim 18,wherein the fugitive polymer comprises polylactic acid.
 22. Apolyacrylonitrile (PAN) precursor according to claim 18, wherein thefugitive polymer comprises a polyester.
 23. A polyacrylonitrile (PAN)precursor according to claim 17, wherein the melt-spinnable PANcomprises AMLON™.
 24. A polyacrylonitrile (PAN) precursor according toclaim 18; wherein the precursor comprises an islands-in-the-seaconfiguration.
 25. A polyacrylonitrile (PAN) precursor according toclaim 24; wherein the precursor comprises an islands-in-the-seaconfiguration such that the PAN comprises a plurality of islands and thefugitive polymer comprises the sea.
 26. A polyacrylonitrile (PAN)precursor according to claim 24; wherein the precursor comprises anislands-in-the-sea configuration such that the fugitive polymercomprises a plurality of islands and the PAN comprises the sea.
 27. Apolyacrylonitrile (PAN) precursor according to claim 24, wherein theweight ratio of the PAN to the fugitive polymer ranges from about 20/80to about 80/20.
 28. A method of forming a continuous, carbon fiber withnanoscale features, comprising: (a) providing a resin mixture comprisingfrom about 50 to about 99.9% of a melt-spinnable-polyacrylonitrile (PAN)and from about 0.01% to about 10% of carbon nanotubes; (b) extruding theresin mixture and feeding the resin mixture into a spin pack assemblycapable of producing fibers comprising nanoscale dimensions andsubstantially aligning the carbon nanotubes with the PAN; (c) oxidizingthe PAN/carbon nanotube fibers by sufficiently heating the fibers; and(d) subjecting the fibers to carbonization comprising the heating of theoxidized fibers to a temperature ranging from about 600 to about 3000°C.
 29. A method according to claim 28, further comprising: (a) providinga fugitive polymer which is dissociable from the melt-spinnable PAN; (b)separately extruding the resin mixture and the fugitive polymer andfeeding both the resin mixture and the fugitive polymer into a spin-packassembly capable of producing multi-component fibers comprisingnanoscale dimensions in an islands-in-a-sea configuration andsubstantially aligning the carbon nanotubes within the PAN; and (c)extracting the fugitive polymer from the multi-component fibers to formPAN/carbon-nanotube fibers.
 30. The method according to claim 29,wherein resin mixture comprises a plurality of islands and the fugitivepolymer comprises the sea.
 31. The method according to claim 30, whereinthe islands comprise solid PAN/carbon nanotube fibers having an outsidediameter ranging from about 20 to about 1000 nanometers.
 32. The methodaccording to claim 30, wherein the islands comprise hollow PAN fibershaving a wall thickness ranging from about 10 to about 500 nanometers.33. The method according to claim 30, wherein the islands comprisehollow PAN fibers having an outside diameter ranging from about 100 toabout 900 nanometers.
 34. The method according to claim 29; wherein theresin mixture comprises the sea and the fugitive polymer comprises aplurality of islands.
 35. A composite comprising layers of sheetscomprising continuous carbon fibers with nanoscale features according toclaim
 1. 36. A prepreg comprising a cloth comprising carbon fibers withnanoscale features according to claim
 1. 37. A perform comprising acloth comprising carbon fibers with nanoscale features according toclaim
 1. 38. A polyacrylonitrile (PAN) precursor, comprising: (a) about50% to about 99.9% by weight of a PAN; and (b) about 0.01% to about 10%of carbon nanotubes.
 39. A polyacrylonitrile (PAN) precursor accordingto claim 38, wherein the PAN comprises a solution-spinnable or agel-spinnable PAN.
 40. A polyacrylonitrile (PAN) precursor according toclaim 39, further comprising a fugitive polymer which is dissociablefrom the solution-spinnable PAN.
 41. A polyacrylonitrile (PAN) precursoraccording to claim 40; wherein the precursor comprises anislands-in-the-sea configuration such that the PAN comprises a pluralityof islands and the fugitive polymer comprises the sea.
 42. Apolyacrylonitrile (PAN) precursor according to claim 40; wherein theprecursor comprises an islands-in-the-sea configuration such that thefugitive polymer comprises a plurality of islands and the PAN comprisesthe sea.
 43. A polyacrylonitrile (PAN) precursor according to claim 38,wherein the PAN comprises a pitch material.
 44. A polyacrylonitrile(PAN) precursor according to claim 43, further comprising a fugitivepolymer which is dissociable from the pitch material.
 45. Apolyacrylonitrile (PAN) precursor according to claim 44; wherein theprecursor comprises an islands-in-the-sea configuration such that thepitch material comprises a plurality of islands and the fugitive polymercomprises the sea.
 46. A polyacrylonitrile (PAN) precursor according toclaim 44; wherein the precursor comprises an islands-in-the-seaconfiguration such that the fugitive polymer comprises a plurality ofislands and the pitch material comprises the sea.
 47. A method offorming a continuous, carbon-nanofiber structure, comprising: (a)providing a resin mixture comprising from about 50 to about 99.9% of apolyacrylonitrile (PAN) and from about 0.01% to about 10% of carbonnanotubes; (b) extruding the resin mixture and feeding the resin mixtureinto a spin-pack assembly capable of producing fibers comprisingnanoscale dimensions and substantially aligning the carbon nanotubeswithin the PAN; (c) oxidizing the PAN/carbon nanotube fibers bysufficiently heating the fibers; and (d) subjecting the fibers tocarbonization comprising the heating of the oxidized fibers to atemperature ranging from about 600 to about 3000° C.
 48. A methodaccording to claim 47, wherein the PAN comprises a solution-spinnablePAN.
 49. A method according to claim 48, further comprising: (a)providing a fugitive polymer which is dissociable from thesolution-spinnable PAN; (b) separately extruding the resin mixture andthe fugitive polymer and feeding both the resin mixture and the fugitivepolymer into a spin-pack assembly capable of producing multi-componentfibers comprising nanoscale dimensions in an islands-in-a-seaconfiguration and substantially aligning the carbon nanotubes with thePAN; and (c) extracting the fugitive polymer from the multi-componentfibers to form PAN/carbon-nanotube fibers.
 50. The method according toclaim 49, wherein resin mixture comprises a plurality of islands and thefugitive polymer comprises the sea.
 51. The method according to claim49; wherein the resin mixture comprises the sea and the fugitive polymercomprises a plurality of islands.
 52. A method of forming a continuous,carbon-nanofiber structure, comprising: (a) providing a resin mixturecomprising from about 50 to about 99.9% of a pitch material and fromabout 0.01% to about 10% of carbon nanotubes; (b) extruding the resinmixture and feeding the resin mixture into a spin-pack assembly capableof producing fibers comprising nanoscale dimensions and substantiallyaligning the carbon nanotubes with the pitch material; (c) oxidizing thepitch/carbon nanotube fibers by sufficiently heating the fibers; and (d)subjecting the fibers to carbonization comprising the heating of theoxidized fibers to a temperature ranging from about 600 to about 3000°C.
 53. A method according to claim 52, further comprising: (a) providinga fugitive polymer which is dissociable from the pitch material; (b)separately extruding the resin mixture and the fugitive polymer andfeeding both the resin mixture and the fugitive polymer into a spin-packassembly capable of producing multi-component fibers comprisingnanoscale dimensions in an islands-in-a-sea configuration andsubstantially aligning the carbon nanotubes with the pitch material; and(c) extracting the fugitive polymer from the multi-component fibers toform pitch/carbon-nanotube fibers.
 54. The method according to claim 52,wherein resin mixture comprises a plurality of islands and the fugitivepolymer comprises the sea.
 55. The method according to claim 52; whereinthe resin mixture comprises the sea and the fugitive polymer comprises aplurality of islands.